Process of removing coloring matter from wood pulp



Patented Mar. 23, 1937 PATENT OFFICE PROCESS or REMOVING coLom'NG MATTERFROM WOOD PULP Georg Jayme, Hawkesbury, Ontario, Canada, as-

signor to Canadian International Paper Company, Hawkesbury, Ontario,Canada No Drawing. Application July 16, 1934, Serial No. 735,438

11 Claims.

This invention relates to the removal of coloring matter from wood pulpand it is particularly directed to the actual removal from wood pulp ofthose materials which are responsible for or cause undesired colors inthe pulp or the products produced therefrom.

Ordinary wood pulps, bleached or unbleached, contain materials ofvarying character, such as waxes, resins, fats, pigments, ligninresidues, etc., and these constitute impurities which are more or lesscolored. While the actual percentages of such impurities present may bevery small they are able to produce or cause undesirable effectsaltogether out of proportion to the percentages 15 of the materialspresent in the pulp. Such materials are largely responsible for theso-called "pitch troubles in thepaper mill and they confer a dark colorto rayon, 'films, cellulose derivatives and like materials manufacturedfrom the pulp. Perhaps the problem created by these impurities is bestillustrated by the fact that their presence prevents the use ofotherwise suitable pulp as a raw material for the manufacture ofcellulose derivatives, as a filler in the plastics industry, or forconversion into papers of high color stability such as high gradewallpapers and the like.

A preferred method of determining the amount of such coloring matter ina pulp is provided by molding the pulp with a colorless resin andcomparing the same colorimetrical ly with a standard similarly preparedby molding purified cotton linters or a wood pulp completely free fromcoloring matter. Pulps which appear perfectly white to the eye and whichyield no material when extracted with organic solvents frequentlydisplay undesired color when so molded.

Impurities very similar to these are found in cotton and in thepurification of the latter alka- 40 line solutions have been employedfor dissolving or dispersing the impurities under heated conditions. Itis also known that the addition, of

' emulsifying agents such as soaps, Turkey red oil and the likesubstantially enhances the dispersing power of thealkaline reagents.This principle of purification has been adopted as a step in thestandard procedure for preparation of chemically pure cellulose forscientific investigations.

Application of such a method of purification to wood pulp has heretoforenot been commercially possible, since proposals heretofore made requirea high percentage, say 3%, of soap calculated on fibre weight togetherwith substantial amounts of caustic soda applied at temperatures ofabout C. and for a period of say, four to six hours. With lesser amountsof reagents correspondingly less of the impurities are removed. Soap isrela-. tively expensive and its cost, apart from that of caustic andheat, makes the known application of 5 this method to wood pulpimpracticable from a commercial point of view.

The chief object of the present invention is thus to provide 'a methodwhereby these impurities may be economically and efiiciently re- 10moved from wood pulp in commercial practice with a view to making suchpulp available for further uses as previously indicated.

In accordance with the invention the pulp at low density is impregnatedwith an emulsifier in solution containing an excess of caustic soda orthe like, then concentrated to high density, in which condition it ismaintained at elevated temperature for completion of the reaction, andthen thoroughly washed. The method is preferably carried out in acontinuous or cyclic manner in which the impregnation liquor isrecovered, strengthened as required, and reused for impregnating furtherquantities of pulp.

Suitable emulsifiers are those which dissolve to practically clearsolutions in dilute caustic soda at ordinary temperatures, say up to 25C. Oleic acid, linoleic acid, sulphonated oils, sulphonated fattyalcohols and the like are suitable but eco-' nomically oleic acid ispreferred. Ordinary soaps are not advantageous. They dissolve to clearsolutions only in hot water and on cooling coagulation of colloidalmasses occurs. This renders the handling of soap solutions in the milldifficult, as pipe lines, storage tanks and the like are not easily keptfree from deposits, particularly during shut downs or interruption ofthe operation. Furthermore, dilute soap solutions are inclined toundergo hydrolytic splitting, precipitating insoluble fatty acidcompounds on or within the fibres of the pulp.

The impregnation liquor containing the emulsifier should have an excessof free caustic soda which enhances the dissolving and dispersing powerof the emulsifier and maintains an equilibrium in the liquor during itsuse. Some of this caustic soda disappears owing to the acid character ofsome of the impurities dissolved from the pulp. Before reusing theliquor caustic soda and emulsifier is added to renew the'liquor andcompensate for what has been removed in first use. The liquor reaches astateof substantially stable equilibrium after several repeated usages,in which condition it may be used to best advantage.

It is not advisable to raise the free causticsoda content of the usedliquor to that' of the fresh liquor, but rather to add only so muchcaustic soda as has been carried away by the pulp. Thus in thesuccessively repeated reuse and renewing 5 of the liquor, there will beremoved in each impregnation only so much of the caustic soda as wasnewly added. Itmay be noted that the content of coloring matter in thecontinuously reused liquor reaches a somewhat similar equilibrium. Forexample, for the first impregnation the liquor may contain 3-50% ofsodium oleate and 3-50% of caustic soda calculated on fibre weight,

depending upon the character of the pulp to be treated, the densityduring impregnation and the extent of purification desired. Assuming,for example, that 10% of the liquor is carried away in the pulp, then0.3-5.0% of sodium oleate and 03-50% of NaOH should be added before orduring reuse of the 90% of the recovered liquor. In the continuousoperation of theprocess the liquor in circulation is kept well balanced.The pulp under impregnation should have a slightly higher consistencythan the pulp concentrated after impregnation, in order to compensatefor the volume of solutions added in renewing the liquor. It is apparentthat the higher the density to which the pulp is concentrated afterimpregnation the smaller will be the loss of chemical reagents. I

After the impregnated pulp has been concentrated it is subjected to amild heat treatment, usually by the introduction of steam. Normally atemperature of to 75 C. maintained for onehalf to two hours issuflicient to permit the reac- 35 tion to proceed to the desired point.The conditions will of course vary with the character of the coloringmatter remaining in the pulp after impregnation. The pulp is then washedfrom the liquor thus removing the rest of the coloring mat- 40 ter. Theuseof warm water, usually available in the mill, reduces the amount ofwash water required. In some cases it is preferable to have the washwater slightly alkaline and for this purpose there may be added thereto,for example, not more than 0.5% of NaOH or the like calculated on theweight of the pulp being washed. The wash water should be low in lime,magnesium,

and the like and preferably free therefrom.

It is desirable that the lime content of the pulp to be treated be low,particularly if the emulsifying agent used forms, with lime, salts-whichare. insoluble or difilcultly soluble or dispersible. U nder theseconditions the lime content of the pulp is preferably not more thanabout 0.02%. With such a pulp the impregnating liquor, containing sodiumoleate as the emulsifier, withdrawn after impregnation remainspractically clear whereas with a relatively high lime bearing pulp itbecomes decidedly turbid. Low lime pulps may be closed in copendingapplication Serial Number 735,437, filed July 16, 1934. It should bementioned that some emulsifiers, such as sulphonated fatty alcohols,form calcium salts which are soluble and when using such emulsifiers theabsence of lime in the pulp is not of particular importance. I

One manner of carrying out the invention in solution, containing theemulsifier and caustic soda or the like, and if necessary water, ismixed with the pulp reducing its consistency to about readily preparedfor example by the method dis- 3%. The low density pulp then enters amixing tank where it is kept in agitation to make the impregnationcomplete and. uniform. The size of this mixing tank need not be greaterthan to provide for a total impregnation time of about 30 minutes. Thepulp mixture is conveyed to another thickener where its density israised to a point slightly lower than the original density beforemaceration. The waste liquor is returned to the mix box where with addedstrengthening materials it is ready for reuse. pulp passes to a mixerwhere the desired temperature is obtained by the introduction of steamor'otherwise as desired and thence to a container where it remains untilthe reaction is complete when it is removed from the container andthoroughly washed. Obviously such a system is quite flexible and themethod may be carried out in various ways. By means of weirs and thelike the addition of chemicals may be easily controlled and the volumeof liquor kept in balance.

The following specific examples are given as further illustrative of theoperation of the invention.

(1) An unbleached strong sulphite pulp, containing objectionableimpurities amounting to 1.4% as indicated by extraction with organicsolvents, was impregnated at a density of 3% with a previously used andstrengthened liquor containing 5% of neutralized'Turkey red oil and 15%of NaOH, both calculated on fibre weight, for five minutes at ordinarytemperature and with thorough agitation or mixing. The pulp wasconcentrated to 25% density, the removed liquor being returned forreuse, and the temperature of the concentrated pulp was brought up to 40C. by the admission of steam to the mixer, at which temperature the pulpwas allowed to remain forv 30 minutes. After washing the pulp free fromimpregnating liquor it was subjected to a con;- ventional two-stagebleach, the second stage he'- ing carried out at a pH higher than 8.0.After washing with slight acidification the refined pulp was very whiteand developed only slight yellow coloration upon molding with acolorless resin. It contained not more than .25% of material extractableby organic solvents. In addition to its improved color other valuablephysical properties of the pulp were better owing to the greater ease ofbleaching. The pulp was particularly suitable for conversion intowallpapers and the like on account of its high degree of color stability.

(2) A soft sulphite pulp partly bleached containing not more than .02%Geo and about .9%

. material extractable with organic solvents was impregnated at adensity of 1.5% with a previously used strengthened liquor containing 8%sodium oleate and 6% caustic soda calculated on the pulp for fifteenminutes with thorough mixing. The pulp was concentrated on a wet machineinto laps of 30% density, the waste liquor being returned for reuse, andthe moist laps were brought to a temperature of 65 C. for 30 minutes,

The concentrated pared from unbleached pulp by chlorination in thepresence of sodium. chloride and not containing more than .01% CaO and1.1% of material extractable with organic solvents was impregnated at 1%density with a previously used liquor containing 12% sodium oieate and60% caustic soda, calculated on fibre weight, for a period of 15 minutesunder normal temperature and with thorough mixing. The pulp was con- 10centrated to a density of 30%, containing in this condition 28% ofsodium oleate and 1.91% caustic soda which are lost during washing, theremoved liquor being returned to the cycle for reuse. The concentratedpulp'was heated to 65? C. for 90 minutesi'and after washing was bleachedwith a soda bleach liquor at a pH of above 8.5 to complete whiteness. Itwas then washed, slightly acidified to remove traces of caustic andrewashed. The bleached pulp contained substantia1ly 90% alpha celluloseand its color was stable -under the severest light treatment. A moldedproduct prepared from it and a suitable resin,

' discoloring effect of impurities tends to impair physical propertiesof the pulp by lowering the alpha cellulose content and otherwise. Byactu ally removing the impurities much milder bleachmg is required andthe damage to the fibre is thus a reduced. Indeed the last stage ofbleaching is not always required with thepresentimethod. The cost ofbleaching material and expense is greatly reduced. 40 A pulp of theinvention having an alpha-cellulose content of say 91% will providerayon the equivalent of that produced from known pulps having anaiphacellulose content of 94 or 95%. It will produce an unbleached rayonas white as a bleached rayon produced from ordinary puips now in use.Thus in the production of rayon from this pulp less handling is requiredand a larger proportion of number one grade material is obtained. Iclaim:

l. A method of refining wood pulp which com-.

prises treatingthe pulp at low density and at ordinary temperaturewith'a liquor containing an emulsifying agent and, an alkali, thickeningthe pulp to high density, retaining thethickened pulp at a raisedtemperature to dissolve or disperse impurities and washing the pulp. V

2. A method as defined in claim 1 wherein the washed pulp is bleachedand then washed with slightly acidified water.

3. A method as defined in claim 1 wherein the operations are carried outcontinuously and the liquor removed upon thickening the pulp is returnedfor reuse.

4. A method as defined in claim 1 wherein impregnation is eiiected at apulp density of .5 to 5%, the pulp is thickened to a density of 15 to30% and is then maintained-at a temperature of 35' to 75 C. for a periodof from onequarter to three hours.

5. A method as defined in claim 1 wherein the pulp is washed with a warmwater containing substantially .5% caustic soda calculated on the basisof the fibre weight of the pulp.

6. A method as 'deflned in claim 1 wherein the treating liquor containsan excess of alkali.

7. A continuous method of purifying wood pulp which comprisesimpregnating the pulp at low density and at ordinary temperature with aliquor containing an emulsifying agent and an excess of caustic soda,thickening the pulp to high density, returning the used liquor forreuse, subjecting the thickened pulp to a temperature not in excess of75 C. to dissolve and disperse remaining impurities and washing-thepulp.

8. A continuous method as defined in claim 7 wherein the usedimpregnating liquor is strengthened in emulsifying agent and causticsoda for reuse.

9. A method of purifying low lime wood pulp which comprises impregnatingthe pulp at low density and ordinary temperature with a liq-' uorcontaining sodium oleate and-an excess of caustic soda, thickening thepulp to a density up to 30%, recovering the impregnating liquor, raisingthe temperature ofthe thickened pulp to about C. to dissolve anddisperse impurities and washing the pulp.

10. A method as defined in claim 7 wherein

